Hexafluoropropylene oxide is an important compound in the production of fluorine-containing compounds, since it is used, for example, as a raw material for perfluorovinyl ether. An oligomer of the hexafluoropropylene oxide is utilized as a lubricating oil, a heating medium and the like.
Generally, hexafluoropropylene oxide is produced by using hexafluoropropylene as a raw material and oxidizing the hexafluoropropylene according to various methods (see, for example, Patent Literature 1).
The reaction mixture obtained in such a production method contains HFPO as the objective product, and the unreacted HFP. It is preferable to purify the reaction mixture to obtain high purity HFPO, and it is also preferable to recover the unreacted HFP to reuse it as a raw material for the production of HFPO.
Distillation is generally utilized for purification. However, boiling points of HFP and HFPO are −29.4° C. and −27.4° C. (both under the atmospheric pressure), respectively. Due to closeness of the boiling points, it is difficult to separate them from each other by distillation. There is proposed a method in which distillation is performed at a low temperature under a low pressure (see Patent Literature 2). However, there is a drawback that a relative volatility cannot be sufficiently increased even by such a method.
Therefore, extractive distillation is utilized so as to separate HFPO from a mixture of HFP and HFPO (see Patent Literatures 1 and 3).
It is known that this extractive distillation may use, as a solvent, a chlorine-containing compound represented by the following general formula (Y):Cn′Ha′Clb′Fc′  (Y)    wherein n′, a′, b′ and c′ are integers which satisfy: n′=2 to 6, 1≦a′≦n+1, 1≦b′≦2n, 1≦c′≦2n, and a′+b′+c′=2n+2.
Specific examples thereof include 1,1-dichloro-1-fluoroethane (HCFC-141b), 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123), 1,2-dichloro-1,1,2-trifluoroethane (HCFC-123a), 3,3-dichloro-1,1,1,2,2-pentafluoropropane (HCFC-225ca) and 1,3-dichloro-1,1,2,2,3-pentafluoropropane (HCFC-225cb).
It is also known that the above extractive distillation may also use, as the solvent, chlorine-containing compounds such as CH2Cl2, CHCl3, CCl4 and CH2ClCH2Cl.